1. Field of the Invention
The present invention relates to a steam inlet portion of a steam turbine into which high-temperature steam flows as a working fluid, and more particularly to a steam turbine which has its respective component parts made of a suitable heat-resisting steel.
2. Description of the Related Art
Energy saving of the thermal power plant is being performed vigorously after the energy crisis, and technology for suppression of the emission of CO2 has being attracting attention in view of the global environmental protection in these years. As part of it, needs for a highly efficient plant are increasing.
To increase power generation efficiency of the steam turbine, it is very effective to raise the turbine steam temperature to a high level, and the thermal power plant having the steam turbine has its steam temperature raised to 600° C. or more in these years. There is a tendency that the steam temperature will be increased to 650° C., and further to 700° C. in future.
In a case where the steam turbine having high-temperature steam of exceeding 650° C. as a working fluid has its respective portions formed of the same materials as the conventional materials, the steam turbine cannot withstand the high-temperature steam of exceeding 650° C., and it is necessary to use a heat-resisting material for the portions which are exposed to the high-temperature steam or to cool such portions. Besides, it is necessary to connect a turbine casing and a steam pipe at a turbine inlet portion through which the high-temperature steam is introduced into the steam turbine, but the turbine casing and the steam pipe are sometimes formed of different materials at such a portion. If materials having a large difference in coefficient of linear expansion are used to form the turbine casing and the steam pipe, there is a drawback that a large thermal stress is generated in the material-welded portion as a steam temperature increases.
Here, as the materials for the outer casing of a conventional 600° C. class steam turbine, there were used, for example, a cast steel which contained C: 0.05 to 0.15, Si: 0.3 or less, Mn: 0.1 to 1.5, Ni: 1.0 or less, Cr: 9 or more and less than 10, V: 0.1 to 0.3, Mo: 0.6 to 1.0, W: 1.5 to 2.0, Co: 1.0 to 4.0, Nb: 0.02 to 0.08, B: 0.001 to 0.008, N: 0.005 to 0.1, Ti: 0.001 to 0.03 and the balance of Fe and unavoidable impurities, and a heat-resisting cast steel which contained C: 0.12 to 0.18, Si: 0.2 to 0.6, Mn: 0.5 to 0.9, Cr: 1.0 to 1.5, Mo: 0.9 to 1.2, V: 0.2 to 0.35, Ti: 0.01 to 0.04 and the balance of Fe and unavoidable impurities, and the unavoidable impurities were suppressed to P: 0.02 or less, S: 0.012 or less, Al: 0.01 or less, Ni: 0.5 or less and Cu: 0.35 or less. And, as a piping material, for example, there was used a heat-resisting steel which contained C: 0.08 to 0.12, Si: 0.2 to 0.5, Mn: 0.3 to 0.6, Cr: 8.0 to 9.5, Mo: 0.85 to 1.05, V: 0.18 to 0.25, Nb: 0.06 to 0.10, N: 0.03 to 0.07, and the balance of Fe and unavoidable impurities, and the unavoidable impurities were suppressed to P: 0.02 or less, S: 0.01 or less and Al: 0.04 or less. And, the material for the outer casing and the piping material were connected to configure the steam inlet portion of the steam turbine. The piping material is a material corresponding to HI STPA28 described in Interpretation-classified Table 1 of Codes for Thermal Power Generation Facilities.
To suppress the generation of a large thermal stress in the above described welded portion, to secure the strength in the welded portion between the steam pipe and the turbine casing and to prevent high-temperature oxidation, it is general to apply materials not having a large difference in coefficient of thermal expansion to the turbine casing and the steam pipe or to lower the temperatures of the materials by cooling the circumference of the welded portion with low-temperature steam as described in, for example, JP-A Hei 8-277703 (KOKAI), JP-A Hei 6-137110 (KOKAI), JP-A Hei 9-32506 (KOKAI), JP-A Hei 11-229817 (KOKAI) and JP-A 2001-65308 (KOKAI).
For example, as a conventional steam turbine employing a manner of lowering the temperature of the materials by cooling the circumference of the welded portion, a low Cr steel can be used for a steam turbine, into which steam at a level of 593° C. flows, by flowing cooling steam to the steam inlet portion as described in, for example, JP-A Hei 8-277703 (KOKAI). Technology of enhancing the cooling effect by producing a swirl flow of cooling steam is also as described in, for example, JP-A Hei 11-229817 (KOKAI) and JP-A 2001-65308 (KOKAI).
It is expected that the steam turbine which is installed in the thermal power system has a tendency to have higher temperatures of main steam and reheated steam in order to obtain high power generation efficiency in the future. For example, in a case where a steam turbine in which a steam temperature exceeds 650° C. is realized using the same material as those of a related art for the individual portions of the steam turbine, the steam turbine cannot withstand the high-temperature steam. Therefore, it is advisable to use heat-resisting materials such as a Ni-base alloy and an austenitic material as the materials for the steam turbine, but if such heat-resisting materials are applied to all the component parts of the steam turbine, the production cost becomes high. Besides, it is hard to produce integrally large products such as the turbine casing and the turbine rotor from the above heat-resisting materials.
In view of the above circumstances, it is desirable to suppress the applicable scope of the heat-resisting materials to the necessity minimum, and there is used a method that the heat-resisting materials are applied to only the portions exposed to high-temperature steam of 650° C. or more among the steam turbine component parts, the conventional materials are applied to the other portions, and they are connected. Thus, there can be adopted a method that the Ni-based alloy is used as the steam pipe material for the steam inlet portion for guiding the high-temperature steam to the steam turbine, and the conventional material is used for the other portions as much as possible. But, in a case where the above method is adopted, there is a drawback that a large thermal stress is produced in the welded portion between the steam pipe material and the material for the other portions with the increase of the metal temperatures if a difference in coefficient of linear expansion is large between them.